This invention is generally directed to processes for the preparation of selenium, and more specifically, the present invention is directed to an improved process for preparing high purity selenium by subjecting a selenium ester to a reduction reaction. In one important embodiment of the present invention, selenium of high purity, 99.999 percent, is obtained by reacting selenious acid, selenium oxides, or mixtures thereof with an alcohol, followed by subjecting the resulting separated selenium ester to a reduction reaction. Selenium prepared from such a process can be used for a number of purposes wherein high purity materials are required, including, for example as a photoconductive imaging member in a xerographic imaging system. Additionally, the high purity selenium prepared in accordance with the process of the present invention can be alloyed with other elements, such as arsenic, tellurium, thallium, antimony, bismuth and the like, and the alloy can be incorporated into a xerographic imaging member.
The incorporation of selenium or selenium alloys into xerographic imaging members is well known in the art. These members can be subjected to a uniform electrostatic charge for the purpose of sensitizing the surface of the photoconductive layer, followed by exposure of an image to activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating member, and wherein a latent electrostatic image is formed in the non-illuminated areas. The resulting image may then be developed and rendered visible by depositing thereon toner particles.
Recently, there has been developed layered organic and inorganic photoresponsive devices containing amorphous seleniuim, trigonal selenium, amorphous selenium alloys, halogen doped selenium, halogen doped selenium alloys, phthalocyanines, and the like. One such photoresponsive member is comprised of a substrate, a photogenerating layer containing vanadyl phthalocyanine or trigonal selenium, and a transport layer containing a diamine dispersed in a resinous binder, reference, for example, U.S. Pat. No. 4,265,990.
Selenium, or an alloy containing selenium selected for photoconductive imaging members must be of high purity, that is a purity of 99.999 percent or greater since the presence of impurities has a tendency to adversely affect the imaging properties of the photconductive member including the electrical properties thereof, causing the copy quality obtained from such devices to be relatively poor in comparison to devices wherein selenium of a high purity is used. While processes are presently available for obtaining selenium of high purity, they involve a number of chemical and physical processing steps, and generally high temperature distillations. Accordingly, many of the prior art processes for preparing selenium and selenium alloys of high purity are complex, economically unattractive, and cause environmental contaminations. Also, such processes can be hazardous to the health of individuals in that, for example, volatile oxides are formed during the high temperature distillations, mercury must be removed in a special added step, and the chemical reagents used in the process cannot in many instances be recycled. Additionally, the prior art processes result in selenium products of different electrical properties despite adherence to the same process conditions.
One present common commercial method utilized for the preparation of high purity selenium involves the formation of selenious acid H.sub.2 SeO.sub.3, from crude selenium, followed by purification, and a complicated and repeated ion-exchange process. The selenium precipitate is then further purified, melted, and subjected to distillation at relatively high temperatures, ranging from about 600 degrees Centigrade to 700 degrees Centigrade, followed by vacuum distillation. The distillation requires very complex and costly equipment, and further, any pollution products such as vaporous oxides and mercury must be safely eliminated, as indicated hereinbefore. Also, this prior art process involves a number of complex steps, and any undesirable waste materials produced must be discarded.
There is disclosed in U.S. Pat. Nos. 4,007,255 and 4,009,249 the preparation of stable red amorphous selenium containing thallium, and the preparation of red amorphous selenium. In the U.S. Pat. No. 4,007,255 there is disclosed a process for producing an amorphous red selenium material containing thallium which comprises precipitating selenious acid containing from about 10 parts per million to about 10,000 parts per million of thallium dioxide, with hydrazine, from a solution thereof in methanol or ethanol, containing not more than about 50 percent by weight of water, at a temperature between about -20 degrees Centigrade and the freezing point of the solution, and maintaining the resulting precipitate at a temperature of about -13 degrees Centigrade to about -3 degrees Centigrade until the solution turns to a red color. The U.S. Pat. No. 4,009,249 contains a similar disclosure with the exception that thallium is not contained in the material being treated.
In addition to the above-described methods for preparing selenium there are known a number of other processes for obtaining selenium and selenium alloys. Thus, for example, there is disclosed in U.S. Pat. No. 4,121,981 an electrochemical method for obtaining a photoreceptor comprised of a selenium tellurium layer. More specifically there is described in this patent the formation of a photogenerating layer by electrochemically co-depositing selenium and tellurium onto a substrate from a solution of their ions in such a manner that the relative amounts of selenium and tellurium which are deposited are controlled by their relative concentrations in the electrolyte, and by the choice of electrochemical conditions.
Accordingly, there is a need for improved processes for preparing selenium in high purity. Additionally, there continues to exist a need for improved simple low temperature chemical processes for preparing selenium and selenium alloys in high purity. There also continues to be a need for improved processes for obtaining selenium in high purity which processes involve a minimum number of process steps, do not require repeated ion-exchange treatments, or high temperature distillations 600-700 degrees Centigrade, and wherein most of the chemical reagents can be recycled and reused. Additionally, there continues to be a need for improved processes for preparing selenium in high yield which processes eliminate environmental hazards associated with the formation and removal of harmful materials. Also, there continues to be a need for improved processes for preparing high purity selenium that has consistent electrical properties.